Differential aggregation properties of secretory proteins that are stored in exocrine secretory granules of the pancreas and parotid glands

Low-pH-and calcium-induced aggregation of regulated secretory proteins has been proposed to play a role in their retention and storage in secretory granules. However, this has not been tested for secretory proteins that are stored in the exocrine parotid secretory granules. Parotid granule matrix proteins were analyzed for aggregation in the presence or absence of calcium and in the pH range of 5.5 to 7.5. Amylase did not aggregate under these conditions, although <10% of parotid secretory protein (PSP) aggregated below pH 6.0. To test aggregation directly in isolated granules, rat parotid secretory granules were permeabilized with 0.1% saponin in the presence or absence of calcium and in the pH range of 5.0 to 8.4. In contrast to the low-pH-dependent retention of amylase in exocrine pancreatic granules, amylase was quantitatively released and most PSP was released from parotid granules under all conditions. Both proteins were completely released upon granule membrane solubilization. Thus neither amylase nor PSP show low-pH-or calcium-induced aggregation under physiological conditions in the exocrine parotid secretory granules.